Not Applicable
The present invention is related to the field of power circuits, and more particularly to power supply circuits using piezoelectric transformers to supply power to fluorescent lamps.
Recent advances in ceramics technology have yielded a new generation of so-called xe2x80x9cpiezoelectric transformersxe2x80x9d (also referred to herein as xe2x80x9cpiezo transformersxe2x80x9d) that are useful in certain applications. These devices, which are constructed using laminated thin layers of ceramic material, exploit a well-known phenomenon called the xe2x80x9cpiezoelectric effectxe2x80x9d to provide AC voltage gain, in contrast to the magnetic field effects relied upon by conventional wound transformers. Like conventional transformers, piezo transformers are fairly rugged and can be used to obtain voltage gain in high-voltage applications. Additionally, due to their thin profile, piezo transformers can be used in applications where bulkier wire-wound transformers are impractical. For example, piezo transformers are used in power supplies that provide high-voltage power to fluorescent lamps used as backlights in portable computers. Due to their thin profiles, piezo transformers used in such applications do not adversely affect the desired sleekness of the portable computer enclosure.
Piezo transformers have recommended operating voltage ratings, arising in part from their ceramic construction. If the input and/or output voltage of a piezo transformer is not within the ratings of the device, then undesirable conditions such as unstable operation, overheating, or failure of the piezo transformer may result. It is therefore important that power supply circuits using piezo transformers comply with these operating voltage ratings.
Piezo transformers operate most efficiently when operated at frequencies at or near a multiple of a fundamental resonant frequency, which is a function of mechanical characteristics of the transformer such as material type, dimensions, etc. However, piezo transformers are high-impedance devices, and therefore their resonance characteristics as well as other characteristics are sensitive to the loading of the transformer output in operational circuits. Resonant frequency, voltage gain at the resonant frequency, and sharpness of the gain-versus-frequency curve all diminish with increased loading.
The diminishing of resonant frequency and gain with an increase in loading are purposely exploited when a piezo transformer is used to drive a fluorescent lamp. The frequency of the signal applied to the primary inputs of the piezo transformer is slowly swept from a frequency higher than the unloaded resonant frequency toward lower frequencies. As the resonant frequency is approached, the gain increases to the point that the transformer output voltage is sufficiently high to xe2x80x9cstrikexe2x80x9d, or initiate conduction in, the lamp. Once the lamp begins conducting, it presents a much higher load to the transformer, causing the voltage gain and therefore the output voltage of the transformer to drop considerably. The conduction characteristics of the lamp are such that it continues to conduct current at the reduced voltage, so the circuit then enters a stable, lower-voltage operating condition. The intensity of the lamp is regulated by controlling the frequency of the AC drive supplied to the piezo transformer as a function of the lamp current.
There are numerous portable computers and other devices in use today, and therefore a number of different configurations of power supply circuits for fluorescent lamps used for backlighting or other purposes. Each unique circuit entails costs associated with design, testing, qualification, fabrication and maintenance. Additionally, each circuit is generally designed to operate with one or at most a limited number of different sets of operating parameters, such as the permissible range of lamp current, the DC voltage from which the power supply circuit obtains power, and other parameters. One circuit may be incapable of operation in other environments, or at best may operate with only low efficiency.
In accordance with the present invention, a control circuit for a piezo transformer based power supply for a fluorescent lamp is disclosed that can be readily adapted to be used in a variety of operating environments, and has features ensuring that optimum efficiency is attained despite the variability of key operational parameters such as DC supply voltage.
The disclosed control circuit includes driver circuitry that supplies respective pulse waveforms to the primary inputs of the piezo transformer, and circuitry for regulating the current in the fluorescent lamp and the voltage across the primary inputs of the piezo transformer. The lamp current regulating circuitry detects the magnitude of current in the fluorescent lamp and varies the frequency of the pulse waveforms generated by the driver circuitry so as to maintain a predetermined desired lamp current, as represented by a predetermined current reference signal. The piezo primary voltage regulating circuitry detects the magnitude of the voltage across the primary inputs of the piezo transformer, and varies the duty cycle of the driver circuitry so as to maintain a predetermined desired piezo primary voltage, as represented by a predetermined voltage reference signal.
In the disclosed system, efficiency is improved by operating the piezo transformer at its optimal gain (i.e., the ratio Vout/Vin) The value of Vout is determined by the magnitude of lamp current, which in turn is determined by the desired lamp intensity. The RMS voltage value at the piezo transformer primary is programmed such that as the system""s DC input voltage is varied (for example from 7 to 22V in the case of a typical notebook computer), the RMS voltage at the piezo transformer primary is held constant. This results in a constant gain and an operating frequency optimized for the piezo transformer. Also, the RMS input voltage to the piezo primary can be programmed to change with lamp load in order to optimize the gain and frequency as the dimming level of the lamp is changed. The piezo transformer can be operated within its recommended operating region despite large variations in the the system""s DC input voltage and/or lamp load.
The disclosed driver circuitry employs four switching transistors arranged as a full bridge with respect to the primary inputs of the piezo transformer. The switching transistors include a first pair for providing a positive pulse to the piezo primary, and a second pair for providing a negative pulse to the piezo primary. Phase shifting circuitry is used to vary the phase of drive signals supplied to the second pair of switching transistors with respect to the phase of drive signals supplied to the first pair of switching transistors so as to maintain the desired voltage across the piezo primary.
A disclosed controller integrated circuit contains a number of components connected to input/output pins such that the integrated circuit can be used in a variety of piezo-based power supply applications.
Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows.